Activation of a synchronous rectifier

ABSTRACT

A rectifier bridge circuit is described for rectifying the phase voltage generated by a generator, including a positive half-bridge having multiple rectifier elements and a negative half-bridge having multiple rectifier elements. The rectifier elements each have a controllable switch having a diode connected in parallel. A control circuit is provided for switching the switches on and off. The switch-on time t switch on setpoint  and/or the switch-off time t switch off setpoint  of the switch is/are computed based on a characteristic map or a mathematical function.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 13/120,619 filed on Jun. 8, 2011, which is anational phase of International Patent Application No.PCT/EP2009/062415, filed on Sep. 25, 2009, and claims priority to GermanPatent Application No. 10 2008 042 352.1, filed on Sep. 25, 2008, thecontents of each of which are hereby incorporated in the accompanyingapplication by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to a rectifier bridge circuit forrectifying the phase voltages generated by a generator for activatingrectifier elements.

BACKGROUND INFORMATION

Rectifiers are used to convert an alternating voltage into adirect-current voltage. Known rectifiers are usually installed in bridgecircuits having multiple diodes as rectifier elements. Common designsare so-called half-bridge or full-bridge rectifiers. In addition, motorvehicle alternating current generators generally include a bridgerectifier for supplying the vehicle electrical system withdirect-current voltage. The rectifier has a power loss which isspecified by the resistance of the diodes and the output current. Thispower loss may be reduced only slightly by using circuitry measures suchas connecting multiple diodes for each phase in parallel, for example.It is therefore known to replace the rectifier diodes with activeswitches, for example MOSFET transistors, via which the power loss maybe significantly reduced. However, the use of active switches requiresactivation of the switches synchronously with the phase frequency. Thepoint in time at which the switches are switched on and off isparticularly critical. Activation of the switches with the aid of acontrol device is relatively complicated and imprecise in the case ofthe known rectifiers.

SUMMARY OF THE INVENTION

An object of the exemplary embodiments and/or exemplary methods of thepresent invention, therefore, is to provide a rectifier bridge circuithaving semiconductor switches which have a simple and robust evaluationelectronics system, and a method for activating the switches of arectifier bridge circuit which operates in a particularly simple andaccurate manner and is functional in variable operating states.

This object is achieved according to the exemplary embodiments and/orexemplary methods of the present invention by the features describedherein. Further embodiments of the present invention are the subjectmatter of the further description herein. The essential concept of thepresent invention is to compute switch-on time t_(switch on) for theparticular switches and/or switch-off time t_(switch off) for the switchbased on a characteristic map or a function. As input variables, thecharacteristic map or the function uses machine-specific parameters suchas rotational speed ng, or rotational speed ng and excitation currentIE, or rotational speed ng and excitation current IE and generatorvoltage UG, or rotational speed ng and excitation current IE androtational angle phi of the rotor. The changes in rotational speedand/or excitation current and/or generator voltage and/or rotationalangle of the rotor may be used as additional input parameters.

Of course, other input parameters which are proportional to the statedvariables may be used instead of the stated variables. For example, thefrequency of the alternating voltage of the phases of the generator, orthe time intervals between the switch-on times for various phases, maybe used instead of generator rotational speed ng. The generator currentor the current in a phase may be used instead of the excitation current.

For controlling the switches, a device for generating control signals isprovided which is supplied with the phase voltages of the generator, therotational speed, and/or the excitation current, and/or the generatorvoltage, and/or the rotational angle of the rotor, and which on thebasis thereof generates the control signals. The switch-on condition ofthe switches is ascertained by measuring the forward voltage of thediode parallel to the switch. This results in commutation of the phasein the optimal “natural” commutation time for the rectifier elements.The switch-off time of the switches may then be based on this point intime, as desired. The control of the switch-on time as a function of theforward voltage has the significant advantage that the control may beprovided in a particularly simple and cost-effective manner using alogic circuit.

The exemplary embodiments and/or exemplary methods of the presentinvention are explained as an example in greater detail below, withreference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows a longitudinal section of an engine alternating currentgenerator for motor vehicles having a claw pole rotor.

FIG. 1b shows the structure of a bridge rectifier having activeswitches.

FIG. 2 shows a three-phase specific embodiment of the engine alternatingcurrent generator, connected in a triangle.

FIG. 3 shows a five-phase specific embodiment of the engine alternatingcurrent generator, connected in a five-pointed star.

FIG. 4 shows a seven-phase specific embodiment of the engine alternatingcurrent generator, connected in a seven-pointed obtuse star.

FIG. 5a shows the basic curve of the phase voltage.

FIG. 5b shows the phase, switch, and diode currents and voltages foractivating the switches according to the present invention.

FIG. 6 shows regulation via t switch off.

FIG. 7 shows regulation via t switch off and a simplified characteristicmap.

FIG. 8 shows regulation via t switch on.

FIG. 9 shows regulation via t switch off.

DETAILED DESCRIPTION

FIG. 1a illustrates a sectional view of an engine alternating currentgenerator 100 for motor vehicles. This engine alternating currentgenerator has, among other elements, a two-part housing 113 composed ofa first end shield 113.1 and a second end shield 113.2. End shield 113.1and end shield 113.2 accommodate a stator 116 having an annularring-shaped core stack 117 which has inwardly open and axially extendinggrooves 119 in which a stator winding 118 is inserted. The radiallyinwardly directed surface of ring-shaped stator 116 encloses anelectromagnetically excited rotor 120 which is designed as a claw polerotor. Rotor 120 is composed, among other elements, of two claw poleplates 122 and 123, on the outer periphery of which claw pole fingers124 and 125 are situated which in each case extend in the axialdirection. Both claw pole plates 122 and 123 are situated in rotor 120in such a way that their claw pole fingers 124, 125 which extend in theaxial direction alternate with one another as north and south poles onthe periphery of rotor 120.

This results in magnetically necessary claw pole interspaces betweenclaw pole fingers 124 and 125 which are magnetized in oppositedirections, and which extend at a slight inclination relative to themachine axis due to pole fingers 124 and 125 which taper toward theirfree ends. Permanent magnets for leakage flux compensation may beintroduced in these claw pole interspaces. This course is referred to ina simplified manner as “axial” for the following description of theexemplary embodiments of the present invention and in the claims. Rotor120 is rotatably mounted in each of end shields 113.1 and 113.2 with theaid of a shaft 127 and one roller bearing 128 situated on each side ofthe rotor. The rotor has two axial end faces, on each of which a fan 130is mounted. These fans 130 are essentially composed of a plate- ordisk-shaped section from which fan blades emerge in a known manner.

These fans 130 are used to allow air exchange between the exterior andthe interior of electric machine 100 via openings 140 in end shields113.1 and 113.2. For this purpose, openings 140 are provided at theaxial ends of end shields 113.1 and 113.2, via which the cooling air isdrawn into the interior of electric machine 100 with the aid of fans130. This cooling air is accelerated radially outward by the rotation offans 130, so that it is able to pass through winding head 145 on thedrive side and winding head 146 on the electronics side, which arepermeable to cooling air. Winding heads 145, 146 are cooled by thiseffect. After passing through winding heads 145, 146 or flowing aroundthese winding heads 145, 146, the cooling air follows a radially outwardpath through openings, not illustrated.

A protective cap 147 which protects various components fromenvironmental influences is shown on the right side in FIG. 1a . Thisprotective cap 147 covers, for example, a slip ring assembly 149 whichsupplies an excitation winding 151 with excitation current. A coolingelement 153, which in this case acts as a cooling element for thecontrolled rectifier/inverter and for the control unit, is situatedaround this slip ring assembly 149. A connecting plate 156 is situatedbetween end shield 113.2 and cooling element 153 which connects thesewinding connection wires to the connections of the control unit and ofthe rectifier/inverter.

FIG. 1b shows the structure of a bridge rectifier having active switcheswhich may be used as the basis in considering the method according tothe present invention. The descriptive text in the circuit is providedfor better understanding. The voltage across at least one or at each ofdiodes D1-D6 is analyzed, for example using a comparator having aswitching threshold of 3.3 volts, for example.

According to the exemplary embodiments and/or exemplary methods of thepresent invention, a control method for generating the control signalsfor a rectifier having active switching elements is described. Thesesignals are obtained, without position sensors and without ahigh-precision analog circuit, by measuring the forward voltage acrossthe diodes with minimal tolerances or by measuring the current. However,in this circuit system it is important that passive diodes are alsopresent parallel to the active switches.

The method or the circuit is therefore particularly suited forrectifiers having active switches which already have these diodes in thesemiconductor design, such as MOSFET transistors, for example. It is notnecessary to introduce additional diodes for these rectifiers. The mainobjective is to provide synchronous rectification in a cost-effectiveand robust manner by simple generation of the control signals, thusreducing the rectifier losses. The exemplary embodiments and/orexemplary methods of the present invention provide an activationstrategy which achieves robust activation and is functional in variableoperating states, using a simple evaluation electronics system.

The switched-on condition of the active switches may be easily detectedby analyzing the voltage across the inverse diode of the MOSFET. For aforward voltage of typically 0.7 V, reliable detection of theswitched-on condition having a limiting value of 0.35 V, for example, ispossible. However, this signal collapses as soon as the activation hasbeen carried out, since the diode forward voltage is bridged by theRDS_on of the MOSFET. This is the basis for the gain in efficiency. In aprior invention it has been proposed to make the switch-off decisionbased on the phase voltages of a subsequent phase or by computation.This is considered to be problematic for the dynamics as the result ofthe rotational speed and the excitation current. It is important togenerate a signal whose time width may be regulated by intentionallyswitching off prematurely in the hysteresis element.

FIG. 2 illustrates, using a circuit diagram, an alternating currentgenerator 100 having three phase-forming phase windings 190, 191, 192.The totality of all phase windings 190, 191, 192 forms stator winding118. The three phase-forming phase windings 190, 191, 192 are connectedto form a basic circuit in the form of a triangle, the connectedwindings in the corners of the triangle defining an angle ofapproximately 60° el. Rectifier/inverter bridge circuit 129 is connectedat the connecting points of corners 200, 201, 202 of the triangle. Thephase windings are connected as follows. Partial phase winding 190 isconnected to partial phase winding 191 at connecting point 200. At itsopposite end, phase winding 191 is connected to phase winding 192 atconnecting point 201. At its opposite end, phase winding 192 isconnected to phase winding 190 at connecting point 202. The connectingpoints may be axially located on or near winding head 146 on theelectronics side in order to achieve the shortest possible connectingpaths. For this purpose, the particular connecting wires of phasewindings 190, 191, 192 of a connecting point 200, 201, 202 to beconnected, which may exit into grooves 119, which are directly adjacent,in the circumferential direction.

Connecting points 200, 201, 202 of phase windings 190, 191, 192 areconnected to a separate controlled bridge inverter-bridge rectifier 119composed of three low-side switches 208 and five high-side switches 209.The number of low-side switches corresponds to the number of high-sideswitches, and corresponds to the number of phase-forming phase windings.The low-side switches and high-side switches may be formed by metaloxide semiconductor (MOS) transistors, bipolar transistors, insulatedgate bipolar transistors (IGPT), or similar switching components. Whenbipolar transistors or IGPTs are used, junction diodes are eachconnected in parallel to the high-side switches and the low-sideswitches, so that the direct current flow directions of the diodes ineach case are correspondingly reversed with respect to the directcurrent flow directions of the switching components.

Power transistors, whose carriers are electrons, may be used astransistors of the high-side switches and low-side switches, since thepower transistors reduce resistance losses and costs; i.e., n-channelMOS transistors are selected in all types of MOS transistors, npntransistors are selected in all types of bipolar transistors, orinsulated gate npn transistors are selected in all types of IGPTs. Acontrol unit which regulates the voltage of the generator by influencingthe current through excitation winding 151 is connected in parallel onthe direct current voltage side. The control unit may also have aconnection to the rectifier in order to ascertain the frequency of thealternating voltage for the voltage induced by the phase-formingwindings, and to ascertain therefrom the instantaneous rotational speedof the engine generator.

The control unit is optionally designed for receiving the rotor positionsignal, the signals from the communication connections, and a controlsignal. The control unit is also operated to generate gate voltagesVG1-VG6 of the particular components, based on the received signals,thus supplying generated gate voltages VG1-VG6 to gate terminals G1-G6of the switching elements in order to switchably control in each case aswitched-on and a switched-off state of same.

FIG. 3 illustrates an alternating current generator 100 having fivephase-forming phase windings 170, 171, 172, 173, 174, using a circuitdiagram. The totality of all phase windings 170, 171, 172, 173, 174forms stator winding 118. The five phase-forming phase windings 170,171, 172, 173, 174 are connected to form a basic circuit in the form ofa five-pointed star, also referred to as a pentagram, the windingsconnected at each point of the star defining an angle of approximately36° el. Rectifier bridge circuit 129 is connected to the connectingpoints of points 180, 181, 182, 183, 184 of the five-pointed star. Thephase windings are connected as follows.

Partial phase winding 170 is connected to partial phase winding 171 atconnecting point 180. At its opposite end, phase winding 171 isconnected to phase winding 172 at connecting point 181. At its oppositeend, phase winding 172 is connected to phase winding 173 at connectingpoint 182. At its opposite end, partial phase winding 173 is connectedto phase winding 174 at connecting point 183. At its opposite end, phasewinding 174 is connected to phase winding 170 at connecting point 184.The connecting points may be axially located on or near winding head 146on the electronics side in order to achieve the shortest possibleconnecting paths.

For this purpose, the particular connecting wires of phase windings 170,171, 172, 173, 174 of a connecting point 180, 181, 182, 183, 184 to beconnected, which may exit into grooves 119, which are directly adjacent,in the circumferential direction. Connecting points 180, 181, 182, 183,184 of phase windings 170, 171, 172, 173, 174 are connected to aseparate bridge rectifier/bridge inverter composed of five low-sideswitches and five high-side switches 159. The remaining designcorresponds to that of FIG. 2. The voltage is measured via a diode, andthe instantaneous rotational speed of the generator is ascertainedtherefrom.

The vehicle electrical system is schematically illustrated by vehiclebattery 30 and vehicle consumer 33.

FIG. 4 shows an advantageous type of connection having sevenphase-forming windings.

A design having 2×3 phases, with the independent three-phase systemselectrically offset with respect to one another, is also possible as astator configuration which meets the intended objective.

FIG. 5a shows the basic curve of the (phase) voltages. The terms havethe following meanings:

“LL_voltage phase Y” indicates the theoretical curve for no-loadvoltage;

“U_Y_diode” indicates the voltage curve when a conventional dioderectifier is used; and

“U_Y_MOSFET” indicates the voltage curve when an ideal switch is used.

Making the switching-off decision is problematic, since in thecontrolled state the battery voltage is applied to the generator. Inaddition, it must be taken into account that the switch-off time maycontinually change due to dynamic changes in the system (rotationalspeed changes, sudden changes in load, changes in the excitation field).

FIG. 5b shows the theoretical curve of the phase no-load voltage withrespect to ground. As an example, the detailed view of the upperhalf-wave is indicated when a comparator between U_BAT and phase Y isused, having a switching threshold of 300 mV.

The terms have the following meanings:

“LL_voltage phase Y” and “B+” denote the curve of the rectified voltageat the terminals of the bridge rectifier. “Activation Y” denotes thesignal for activating the switch;

“Y_diode” denotes the theoretical curve of the phase voltage withrespect to ground for strictly diode operation without using theswitches; and

“Y_MOSFET” denotes the actual curve of the phase voltage with respect toground at the first moment, while “t diode on” denotes that the voltageincreases due to voltage drop Uf across the diode. As soon as theswitched-on condition for the MOSFET is sensed, time segmentt_(diode on) is terminated and time segment t_(switch on) begins. Duringthis time segment the voltage phase with respect to ground is onlyslightly above the generator voltage. At point in time T1 the switch isopened, and the voltage phase with respect to ground increases to thevalue for diode operation. The diode takes over the current for timet_(switch off).

One possible method according to the present invention proceeds asfollows:

At point in time T_0 the activation decision may be made based on therecognition by the comparator. The voltage above the switch collapses.The comparator signal goes to “off.” At point in time T_1 the controllogic system turns the switch off based on a learned pulse-duty factor.The system changes to diode operation, the voltage of the phase exceedsthe comparator threshold, and the comparator signal jumps to “on.” T_2is reached as soon as the system commutates off on its own. On the basisof measured time difference T_2−T_1, a controller may be developed forcontrolled time difference T_2−T_1 which maximizes the active rangebetween T_0 and T_1, and therefore, the efficiency. A characteristic mapwhich is a function of rotational speed n is used as the setpointsetting for T_2−T_1, since in the lower rotational speed range a higherdynamics of the system must be made available. In addition, the lengthof the active window is a function of the excitation current of themachine. This is achieved by regulating window width T_2−T_1. The aim isfor T_2−T_1 never to be zero.

One advantage of the exemplary embodiments and/or exemplary methods ofthe present invention is reliable recognition of the switch-on andswitch-off times of the MOSFET, using only one analog comparator systemper switch which is immune to interference. The timing control as wellas the regulation to a defined, in particular minimal, time T_2−T_1 maybe carried out digitally.

Another main advantage is that an adaptive readjustment of dynamicchanges in the system, in particular speed changes, sudden changes inload, changes in the excitation field, etc., are corrected with the aimof achieving maximum performance with minimum susceptibility tointerference.

FIG. 6 shows the design of a first specific embodiment of the presentinvention. In this case a characteristic map is employed which usesrotational speed ng, or rotational speed ng and excitation current IE,or rotational speed ng and excitation current IE and generator voltageUG, or rotational speed ng and excitation current IE and rotationalangle phi of the rotor as input variables, and which outputs t switchoff setpoint as the output variable. This value is supplied to thecontroller as a setpoint value.

FIG. 7 illustrates a specific form of the first exemplary embodiment,t_(switch off setpoint) being computed from a characteristic map havinggenerator rotational speed ng as the input variable. The controller isdesigned as a PI controller.

FIG. 8 shows the design of a second specific embodiment of the presentinvention. In this case a characteristic map is employed which usesrotational speed ng, or rotational speed ng and excitation current IE,or rotational speed ng and excitation current IE and generator voltageUG, or rotational speed ng and excitation current IE and rotationalangle phi of the rotor as input variables, and which outputs t switch onsetpoint as the output variable. This value is supplied to thecontroller as a setpoint value.

FIG. 9 shows the principle of a third exemplary embodiment. In thiscase, time t switch off is measured. If this value is greater than 0, acontrol algorithm is run which adjusts time t_(switch off) to a valuet_(switch off min). Value t_(switch off min) may be a constant value, oris a percentage of the commutation duration or the duration in theswitched-on state. Value t_(switch off) is less than or equal to 0.

What is claimed is:
 1. A rectifier bridge circuit for rectifying thephase voltage generated by a generator, including a positive half-bridgehaving multiple rectifier elements and a negative half-bridge havingmultiple rectifier elements, the rectifier elements each having acontrollable switch having a diode connected in parallel, and a controlcircuit being provided for switching the switches on and off, whereinthe switch-on time t_(switch on setpoint) and/or the switch-off timet_(switch off setpoint) of the switch is/are computed based on acharacteristic map or a mathematical function.
 2. The rectifier bridgecircuit as recited in claim 1, wherein the characteristic map or themathematical function has as input parameters the generator rotationalspeed ng, or the generator rotational speed ng and the excitationcurrent IE, or the generator rotational speed ng and the excitationcurrent IE and the generator voltage UG, or the generator rotationalspeed ng and the excitation current IE and the rotational angle phi ofthe rotor.
 3. The rectifier bridge circuit as recited in claim 1,wherein the characteristic map or the mathematical function has as inputparameters the generator rotational speed ng and the ratio of theswitched-on duration to the theoretical maximum switched-on duration(commutation time).
 4. The rectifier bridge circuit as recited in claim2 or 3, wherein the characteristic map or the mathematical function hasas additional input parameters the change in the generator rotationalspeed ng, or the change in the generator rotational speed ng and thechange in the excitation current IE, or the change in the generatorrotational speed ng and the change in the excitation current IE and thechange in the generator voltage UG, or the change in the generatorrotational speed ng and the change in the excitation current IE and thechange in the rotational angle phi of the rotor.
 5. The rectifier bridgecircuit as recited in claim 1, 2, 3, or 4, wherein the valuet_(switch on) which is computed from the characteristic map or themathematical function is used within the scope of a control.
 6. Therectifier bridge circuit as recited in claim 1, 2, 3, or 4, wherein acontroller is present which adjusts t_(switch off) based ont_(switch off setpoint) and t_(switch off actual).
 7. The rectifierbridge circuit as recited in one of the preceding claims, wherein thecontroller controls more rapidly at a low rotational speed and/or smallexcitation currents.
 8. The rectifier bridge circuit as recited in oneof the preceding claims, wherein the controller-based switch-on time isset to zero when the switch-on time of the switch exceeds thecommutation time.
 9. The rectifier bridge circuit as recited in one ofthe preceding claims, wherein the derivative action within thecharacteristic maps or the mathematical function is greater for smallerexcitation currents and/or lower rotational speeds.
 10. The rectifierbridge circuit as recited in one of the preceding claims, wherein theswitch-on time of a switch is ascertained using a comparator.
 11. Therectifier bridge circuit as recited in claim 10, wherein the diodeforward voltage is used as the input variable for the comparator, andthe switch is controlled at 0.7 V forward voltage, preferably 0.35 Vdiode forward voltage.
 12. The rectifier bridge circuit as recited inone of preceding claims 1 through 5, wherein a regulation minimizes theswitched-off duration t_(switch off) to a value t_(switch off min). 13.The rectifier bridge circuit as recited in claim 12, wherein thecontroller-based switch-on time is set to a value t_(switch on setpoint)based on the characteristic map or the mathematical function when theswitch-on time of the switch exceeds the commutation time, thederivative action defining the percentage ratio oft_(switch off setpoint) to the total commutation time.